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CENTER OF MASS AND MOMENT OF INERTIA     1
(b) Suppose the square OA to represent a plate of positive mass a the square O'A to represent a plate of negative mass. Then if the t plates have the same thickness and density the positive and the negat: masses annul each other in the square O'A. Therefore the two squ; plates form a system which is equivalent to the actual plate represeni by the shaded area of the figure. Hence the center of mass of the squ; plates is also the center of mass of the given plate.
The masses of the square plates are <r&2 and nates of their centers of mass are
era2, while the coor
-,                  i
= y'=~   and
2b  a
Therefore the coordinates of the center of mass of the two are
T o &  i   /         o\ 2 &  a
= b*-\~ab  a2 2 (a + 6)    '
which are identical with those obtained by the first method.
PROBLEMS.
1.  Find the center of mass of the homogeneous plates indicated by following figures:
(e)
Find
2.   A sphere of radius 6 has a spherical cavity of radius a. center of mass if the distance between the centers is c.
3.   A right cone is cut from a right circular cylinder of the same b and altitude.   Find the center of mass of the remaining solid.
4.   A right cone is cut from a hemisphere of the same base and altitu Find the center of mass of the remaining solid.
6. A right circular cone is cut from another right circular cone of same base but of greater altitude. Find the center of mass of the rems ing solid.
6. A right circular cone is cut from the paraboloid of revolution gei